Alcohols Fischer-Tropsch reactions

The Fischer-Tropsch reaction is essentially that of Eq. XVIII-54 and is of great importance partly by itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, olefins, alcohols, and gasolines. The first step is to produce a mixture of CO and H2 (called water-gas or synthesis gas ) by the high-temperature treatment of coal or coke with steam. The water-gas shift reaction CO + H2O = CO2 + H2 is then used to adjust the CO/H2 ratio for the feed to the Fischer-Tropsch or synthesis reactor. This last process was disclosed in 1913 and was extensively developed around 1925 by Fischer and Tropsch [268]. 

Sasol Fischer-Tropsch Process. 1-Propanol is one of the products from Sasol s Fischer-Tropsch process (7). Coal (qv) is gasified ia Lurgi reactors to produce synthesis gas (H2/CO). After separation from gas Hquids and purification, the synthesis gas is fed iato the Sasol Synthol plant where it is entrained with a powdered iron-based catalyst within the fluid-bed reactors. The exothermic Fischer-Tropsch reaction produces a mixture of hydrocarbons (qv) and oxygenates. The condensation products from the process consist of hydrocarbon Hquids and an aqueous stream that contains a mixture of ketones (qv) and alcohols. The ketones and alcohols are recovered and most of the alcohols are used for the blending of high octane gasoline. Some of the alcohol streams are further purified by distillation to yield pure 1-propanol and ethanol ia a multiunit plant, which has a total capacity of 25,000-30,000 t/yr (see Coal conversion processes, gasification). 

The Fischer-Tropsch reaction has now been known for almost 70 years and is of great importance partly for itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, alkenes, alcohols, and gasolines. According to Eqs.I-IV in the most... 

The hydroformylation of alkenes was accidentally discovered by Roelen while he was studying the Fischer-Tropsch reaction (syn-gas conversion to liquid fuels) with a heterogeneous cobalt catalyst in the late thirties. In a mechanistic experiment Roelen studied whether alkenes were intermediates in the "Aufbau" process of syn-gas (from coal, Germany 1938) to fuel. He found that alkenes were converted to aldehydes or alcohols containing one more carbon atom. It took more than a decade before the reaction was taken further, but now it was the conversion of petrochemical hydrocarbons into oxygenates that was desired. It was discovered that the reaction was not catalysed by the supported cobalt but in fact by HCo(CO)4 which was formed in the liquid state. 

The intrinsic nature of tungsten carbide catalyst in CO-H2 reactions is to form hydrocarbons. This property can be modified by oxidic promoters as for the case of noble metals like Pt or Rh or by the presence of carbon vacancies at the surface. To increase the production of alcohols in the Fischer-Tropsch reaction, the catalyst should be bifunctional, with oxidic and carbidic components as in the case of WC on Ti02. Overcarburization of WC on supports like Si02 or Zr02 where the W-O-metal interaction is weak leads to C/W ratios close to unity and does not result in alcohol formation. 

A mixture of CO + H2 is used in the Fischer-Tropsch reaction to make hydrocarbons in high yields. The reaction requires a catalyst, usually Fe or Ni supported on silica, a temperatue of 200-400°C and a short contact time. Depending on the conditions, aliphatic hydrocarbons, aromatic hydrocarbons, alcohols, and acids can be produced. If NH3 is added to the CO + H2, then amino acids, purines, and pyrimidines can be formed.23 The intermediates in these reactions are not known, but it is likely that HCN is involved together with some of the intermediates postulated for the electric discharge processes. 

In the production of paraffins, the mixture of carbon monoxide and hydrogen is enriched with hydrogen from the water-gas catalytic (Bosch) process, i.e., shift reaction (Fig. 1), and passed over a cobalt-thoria catalyst to form straight chain (linear) paraffins, olefins, and alcohols (Fischer-Tropsch synthesis) ... 

Here the chemical formula is written CH2, which is one-eighth of a typical gasoline molecule (CgHi6). The reaction is catalyzed by a number of metal-based catalysts including iron, cobalt, and nickel. The reactors in which the synthesis takes place operate within a temperature range of 225 to 365°C and at pressures from 0.5 to 4 MPa. It should also be noted that the Fischer-Tropsch reactions produce a wide spectrum of oxygenated compounds such as alcohols. 

Some mechanistic information is available on ruthenium-based homogeneous Fischer-Tropsch reactions. By in situ IR spectroscopy, in the absence of any promoter, only Ru(CO)5 is observed. An important difference between the cobalt and the rhodium system on the one hand and ruthenium on the other is that in the latter case no ethylene glycol or higher alcohols are obtained. In other words, in the catalytic cycle the hydroxymethyl route is avoided. 

The hydroformylation (or 0x0 ) reaction was discovered in 1938 by Roelen who was working on the formation of oxygenates as by-products of the Fischer-Tropsch reaction over cobalt catalysts. It soon became clear that the aldehydes and alcohols found were the products of secondary reactions undergone by the 1-alkenes (which are the primary products of the Fischer-Tropsch reaction, Section 4.7.2) with syngas. Further work showed that Roelen had discovered a new reaction, in which the elements of H and CHO were added to an olefin (hence hydroformylation), and which was catalyzed by cobalt. It was later found that the true precatalyst was not cobalt metal but derivatives of dicobalt octacarbonyl, such as the hydride, CoH(CO)4. 

It is now widely accepted that the activation of CO is highly structure sensitive (II). The activation of CO on most of the transition metals has been investigated. The computational results for cobalt (6) and ruthenium (5) are of particular relevance to us because these elements in the metallic state are active for the Fischer-Tropsch reaction. These results can be compared with those obtained for rhodium (40), which selectively catalyzes the formation of alcohols from CO and H2, and for nickel (30), which is a methanation catalyst. 

Fig. 13. The Fischer-Tropsch reaction produces homologous series in logarithmic distribution, as illustrated here by alcohols (Anders et al, 1974). From C2 upward, successive homo-logues form in fixed, molecular ratio C +t/C = a, where a is the probability of chain growth, typically 0.6-0.9. Interstellar cyanoacetylenes in the Taurus Molecular Cloud 1 likewise show a logarithmic distribution, of very similar slope (0.52 vs 0.59 for the FTT alcohols). Presumably they, too, formed by surface catalysis, rather than by gas-phase reactions...

From reaction (b), a new carbon-carbon bond is formed by the intermediacy of the transition metal. Several chemical processes in which new carbon-carbon bonds are formed (e.g., hydroformylation, olefin polymerization, homologation of alcohols, the Fischer-Tropsch reaction) are rationalized by a common mechanism of carbon chain growing. Much discussion and scientific work is under way to ascertain the general applicability of these findings to specific chemical or biochemical processes however, a basic understanding of the elementary steps involved in insertion reactions will lead to a better understanding of known reactions and to the development of new reactions. 

Carbon monoxide may be hydrogenated to produce either alcohols or hydrocarbons, depending on the catalysts used and the reaction conditions. Temperatures ranging from 100-400°C and pressures as high as 1,000 atm have been studied. Different catalysts yield radically different types of products. Important processes for uch reactions consist of the methanol synthesis, the higher-alcohol synthesis (or the variation termed the iso synthesis), the Fischer-Tropsch reaction (or the version called hydrocarbon synthesis), and the methanation reaction. These syntheses were discovered in the period 1920-1925, at which time the methanol and higher-alcohol syntheses developed rapidly. A brief summary of processes and conditions used for the hydrogenation of carbon monoxide is presented in Table 10-17. 

The synthesis of fatty acids by a Fischer-Tropsch-type process as described in this chapter required the use of a catalyst (meteoritic iron) and a promoter. Potassium carbonate and rubidium carbonate were the only compounds evaluated which unambiguously facilitated the production of fatty acids. These catalytic combinations (meteoritic iron and potassium carbonate or rubidium carbonate) also produced substantial amounts of n-alkenes (in excess of n-alkanes) and aromatic hydrocarbons. A comprehensive study of the nonacidic oxygenated compounds produced in Fischer-Tropsch reactions (20,21) was not made. However, in the products analyzed (all promoted by potassium carbonate), long-chain alcohols and aldehydes were detected. 

Fischer-Tropsch reaction affords various products, viz., saturated hydrocarbons, alkenes, aromatic hydrocarbons, alcohols, aldehydes, ketones, acids, esters, and compounds which are formed by reactions between these products. 

But the TMS catalysts have many other uses as well, including reforming, isomerization of paraffins, dehydrogenation of alcohols, Fischer-Tropsch and alcohol synthesis, hydration of olefins, amination, mercaptan and thiophene synthesis, and direct coal liquefaction. A comprehensive list of the TMS-catalyzed reactions is presented in Reference 3. 

Maitlis has reviewed the homogeneous and heterogeneous systems and noted that the former produce mainly oxygenated products, such as alcohols and esters. Most of the recent work on the Fischer-Tropsch reaction has focused on heterogeneous catalysts. The mechanism(s) of the later ate mainly inferred from isotope labeling and product distributions. The observations and mechanistic proposals of the Sheffield group have been summarized by Maitlis. However, there is still some controversy about the heterogeneous pathways. ... 

Most organic chemicals are currently made commercially iixim ethylene, a product of oil lehning. It is possible that in the next several decades we may have to shift toward other carbon sources for these chemicals as depletion of our oil reserves continues. Either coal or natural gas (methane) can be converted into CO/Hz mixtures with mr and steam (Eq. 12.18), and it is possible to convert such mixtures, variously called water-gas or synthesis gas to methanol (Eq. 12.18) and to allume fuels with various heterogeneous catadysts. In particular, the Fischer-Tropsch reaction (Eq. 12.19) converts synthesis gas to a mixture of long-chain alkanes and alcohols using heterogeneous catalysts. 

Synthesis gas is an important intermediate. The mixture of carbon monoxide and hydrogen is used for producing methanol. It is also used to synthesize a wide variety of hydrocarbons ranging from gases to naphtha to gas oil using Fischer Tropsch technology. This process may offer an alternative future route for obtaining olefins and chemicals. The hydroformylation reaction (Oxo synthesis) is based on the reaction of synthesis gas with olefins for the production of Oxo aldehydes and alcohols (Chapters 5, 7, and 8). 

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